Climate affects predator control of an herbivore outbreak

Herbivore outbreaks and the accompanying devastation of plant biomass can have enormous ecological effects. Climate directly affects such outbreaks through plant stress or alterations in herbivore life-history traits. Large-scale variation in climate can indirectly affect outbreaks through trophic interactions, but the magnitude of such effects is unknown. On the California coast, rainfall in years during and immediately previous to mass lupine mortality was two-thirds that of years without such mortality. However, neither mature lupines nor their root-feeding herbivores are directly affected by annual variation in rainfall. By increasing soil moisture to levels characteristic of summers following El Niño/Southern Oscillation (ENSO) events, we increased persistence of a predator (the entomopathogenic nematode Heterorhabditis marelatus). This led to suppression of an outbreak of the herbivorous moth Hepialus californicus, indirectly protecting bush lupine (Lupinus arboreus). Our results are consistent with the marine-oriented Menge-Sutherland hypothesis (Menge and Sutherland 1987) that abiotic stress has greater effects on higher than on lower trophic levels. The mechanisms producing these results differ from those proposed by Menge-Sutherland, however, highlighting differences between trophic processes in underground and terrestrial/marine food webs. Our evidence suggests that herbivore outbreaks and mass lupine mortality are indirectly affected by ENSO's facilitation of top-down control in this food web.